The present invention relates to a scavenging fringe field for electrostatographic printing machines and more particularly to the use of such as a field to reduce toner contamination and improve copy quality.
In an electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to a support surface, the photoconductive insulating member is cleaned of any residual toner that may remain thereon in preparation for the next imaging cycle.
In an ideal printing machine, the toner particles would be confined to only the image areas on the imaging member, the development zone, the cleaning zone and the copy paper. However, as a practical matter, it is very difficult to control the movement of all toner particles and thus some become airborne throughout the xerographic cavity of the machine and come to light or rest on various surfaces of the machine. This is particularly true for toner particles which may escape from either the cleaner sump or the developer housing or both that may be mechanically carried by the imaging member through the various processing stations. This stray toner which may be electrostatically charged to either positive or negative polarity is electrostatically attracted and is deposited on a large number of surfaces. One of the more troublesome areas where significant quantities of stray toner collect is in the cleaner assembly and in particular on the underside of the cleaning blade in an assembly of the construction such as is illustrated in FIGS. 2A and 2B. When the machine is in operation producing prints, the blade tends to observe a tucked position as illustrated in FIG. 2B. However, after a copying run has been completed and the machine is permitted to cycle down and come to rest, the blade tends to become untucked and relaxes so that its underside tends to drop down on the imaging member so that a significant area of the back of the blade may contact the imaging member and any stray toner present thereon will accumulate on the back of the cleaning blade. In a subsequent printing run, this stray toner and in particular the stray toner on the back of the cleaning blade is in very close proximity to the imaging member and may be deposited thereon eventually resulting in a copy quality defect in the final print produced.
The propensity for deposition of stray or contaminating toner on the imaging member is enhanced by the fact that following transfer of toner to the copy sheet and during stripping of the first print during a printing run during transfer and as the lead edge of the copy paper is separated from the imaging member there is an air breakdown between the imaging member and the copy paper resulting in ionization of the air and creation of an electrostatic fringe field on the imaging member. Typically, this fringe field is a narrow field in the width dimension across the imaging member with a large gradient and is of the order of 300 to 500 volts higher than adjacent non-image areas which are of the order, for example, of 250 volts. As this fringe field created at the transfer station continues to pass through the printing machine, it will attract oppositely charged toner for the most part but may also attract some toner of the same charge on each side of the fringe field. Furthermore, if substantial quantities of toner are attracted from the cleaning blade and other surfaces, the stray toner on the imaging member may end up being charged during the next charging step for a second print. In addition, the imaging member may be not discharged by light during exposure since the light does not reach the imaging member having been blocked by the deposited toner particles. The problem is further compounded by the fact that although most development systems do function to scavenge any residual toner present on the imaging member during the development process simultaneously with development of the new latent image in the image areas, this scavenging process is not very efficient when the toner is held by the field formed at transfer and therefore the development process in ineffective in this scavenging role. Furthermore, in those machine configurations wherein there is a precharge discharge device such as a lamp, the imaging member may not be sufficiently discharged in those areas which contain stray toner since the imaging member does not see the light used for the discharge purposes as the stray toner is blocking the light path to the imaging member. Accordingly, the fringe field created during the stripping process may attract sufficient stray toner that survives the scavenging operation during the development process that it will be present on the imaging member in the transfer of the toner image in the second print made in any copying run.
This problem is particularly troublesome in those machine configurations where a registered system is used. That is, a system wherein the images that are formed on the imaging member are formed on the same area or areas of the imaging member for every print or prints that are made. Thus, successive registered areas of the imaging member or plates are used to provide successive developed toner image prints. This is particularly important in those imaging systems wherein a seamed belt is used as the imaging member. The belt is required to be in registration for all prints produced thereon so that the seam does not show up on any of the prints. Typically, this is controlled by the machine sensing the position of a small hole present on one edge of the belt and timing the printing operation for each print from that hole to ensure registration of the image being formed only in the registered area of the belt. In such a machine configuration, the stray toner deposited in the fringe field created on stripping the first copy sheet from the first registered area may result in a copy quality defect in the second print produced in the same registered area of the imaging member if registration for the second print is advanced relative to the registered area on the imaging member for the first print This can occur because of slight variations in the timing windows and imaging member timing hole sensing. While this copy quality defect may be observed on all prints after the first produced in a registered area it is most frequently observed only on the second copy produced in a registered areas because the first fringe field traverses the path past the processing stations and in particular the cleaning blade and it purges most of the toner from these stations, in particular, the back of the blade. During the formation of subsequent prints on a printing run very small residual quantities of toner are observed at the lead edge of the registered areas on the imaging member which in general do not survive the scavenging mechanism of the developing housing.
These difficulties have been previously minimized by discharging the fringe field formed during stripping before that area of the imaging member reaches the cleaner assembly. This may be accomplished either with a preclean corotron as for example, is used in the Xerox 3300 copier or the use of a preclean exposure to discharge the photoreceptor before cleaning. However, both of these techniques are expensive in that they require additional apparatus either in the form of a corotron or an illumination device. In addition to the initial cost of the devices, maintenance and replacements are additional burdens with which the user must contend.